It is known to use documents having such position identification markings in combination with a pen having an imaging system, such as an infra red camera, within it, which is arranged to image a small area of the page close to the pen nib. The pen includes a processor having image processing capabilities and a memory and is triggered by a force sensor in the nib to record images from the camera as the pen is moved across the document. From these images the pen can determine the position of any marks made on the document by the pen. The pen markings can be stored directly as graphic images, which can then be stored and displayed in combination with other markings on the document. In some applications the simple recognition that a mark has been made by the pen on a predefined area of the document can be recorded, and this information used in any suitable way. This allows, for example, forms with check boxes on to be provided and the marking of the check boxes with the pen detected. In further applications the pen markings are analysed using character recognition tools and stored digitally as text.
Examples of known patterns include lines at variable spacings, such as those disclosed in EP 0 206 246, and dot patterns in which the dot pattern varies in a systematic manner such as those disclosed in, for example, WO 02/082366. It will be appreciated that for these systems to work the imaging system in the pen needs to image an area of the product that is sufficiently large for the pattern it contains to be unique. Also the imaging capability of the pen needs to provide a sufficiently high resolution for the positions of the lines or dots to be identified.
Examples of known patterns involving varying reflectivity are disclosed for example in WO03/001442 or in US2003/104938. The variation on reflectivity is however dependent on environmental conditions such as lighting or on the way the system is used or even on the system itself, so that accurate absolute positioning may lack in precision.